Zoom lens system for use with a compact camera

ABSTRACT

A zoom lens system for use with a compact camera comprises, in order from the object side, a first lens group having a positive focal length and a second lens group having a negative focal length and which performs zooming by changing the distance between the first and second lens groups. The first lens group comprises, in order from the object side, a subgroup 1a of a small power that is composed of a positive first lens and a negative second lens and a subgroup 1b of a large positive power that has a cemented surface of a negative power on the image side. The subgroup 1a has at least one aspheric surface that has a divergent amount of asphericity with respect to a paraxial radius of curvature.

This is a continuation of application Ser. No. 07/589,565 filed Sep. 28,1990, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a zoom lens system that is suitable foruse with a compact camera and which is subject to less constraints onback focus than zoom lens systems for use with single-lens reflexcameras. More particularly, the present invention relates to a zoom lenssystem of a two-group type that is small in size and which can bemanufactured at a lower cost than conventional versions.

Conventional zoom lens systems for use with compact cameras areclassified as two types, (A) a two-group type and (B) a three- orfour-group type. Compared with type (A), zoom lens systems of type (B)have the advantage of requiring a small amount of lens movement but, onthe other hand, they are not only large in size but also complex inconstruction. Because of these obvious differences from lens systems ofa two-group type which are envisaged by the present invention, type (B)will not be described in detail hereinafter.

Compared to type (B), zoom lens systems of type (A) require a somewhatgreater amount of lens movement but because of their simple lensconfiguration and mechanical structure, type (A) zoom lens systems havethe advantage of ease in size reduction. Conventionally known zoom lenssystems of a two-group type include version (A-1) that is described inUnexamined Published Japanese Patent Application Nos. Sho-56-128911,Sho-57-201213, Sho-60-48009, Sho-60-170816 and Sho-60-191216, version(A-2) that is described in Unexamined Published Japanese PatentApplication Nos. Sho-62-90611, and Sho-64-57222, and version (A-3) thatis described in Unexamined Published Japanese Patent Application Nos.Sho-62-113120 and Sho-62-264019.

Version (A-1) has a small back focus and requires a large rear lensdiameter, so it has had the problem that the overall size of the cameraincorporating said lens system cannot be reduced. A further problem withthis lens system is that internal reflections between the film plane andthe last lens surface and other unwanted phenomena are highly likely tooccur.

With a view to solving these problems, the assignee has proposedimproved versions of a two-group type the back focus of which iscomparatively large in consideration of its use with compact cameras.Such improved versions are (A-2) which is of a five-group-six-elementcomposition and which is capable of a zoom ratio of 1.5-1.6 and (A-3)which is of a six-group-seven-element composition orseven-group-eight-element composition and which is capable of a zoomratio on the order of 1.7-2.5. These versions range from a six-elementcomposition capable of a zoom ratio of ca. 1.5-1.6 to an eight-elementcomposition capable of a zoom ratio of at least 2. However, theseproposals have been unable to satisfy the need for providing a zoom lenssystem for use with a compact camera that is even more compact and lessexpensive.

The present invention has been accomplished in order to meet this needof the prior art and its principal object is to provide a zoom lenssystem suitable for use with a compact camera which is basically animprovement over the version (A-2) described in Unexamined PublishedJapanese Patent Application No. Sho-64-57222 in that the achievable zoomratio and the less composition are substantially the same as in (A-2)but which yet is not only smaller in size but also less expensive onaccount of the extensive use of a low-refractive index glass or plasticmaterial.

SUMMARY OF THE INVENTION

The zoom lens system of the present invention for use with a compactcamera comprises basically, in order from the object side, a first lensgroup having a positive focal length and a second lens group having anegative focal length and performs zooming by changing the distancebetween the first and second lens groups. In accordance with a firstaspect of the present invention, said first lens group comprises, inorder from the object side, a subgroup 1a of a small power that iscomposed of a positive first lens and a negative second lens and asubgroup 1b of a large positive power that has a cemented surface of anegative power on the image side; said subgroup 1a has at least oneaspheric surface that has a divergent amount of asphericity with respectto a paraxial radius of curvature is such a way as to satisfy thefollowing condition (5); and said zoom lens system further satisfies thefollowing conditions: ##EQU1##

In accordance with a second aspect of the present invention, either oneor both of the first and second lenses in subgroup 1a is formed of aplastic material; if the first lens is formed of a plastic material, thefollowing condition is satisfied:

    (m.sub.1a-2 ·m.sub.1b ·m.sub.2L).sup.2 <0.3 (6)

if the second lens is formed of a plastic material, the followingcondition is satisfied:

    (m.sub.1b ·m.sub.2L -m.sub.1a-2 ·m.sub.1b ·m.sub.2L).sup.2 <0.6                            (7)

if both the first and second lenses are formed of a plastic material,the following conditions is satisfied:

    (m.sub.1b ·m.sub.2L).sup.2 <0.3.                  (8)

In accordance with a third aspect of the present invention, the secondlens group comprises, in order from the object side, a positive meniscuslens 2-1 having a convex surface directed toward the image and anegative meniscus lens 2--2 having a concave surface directed toward theobject, and the zoom lens system further satisfies the followingadditional conditions: ##EQU2##

In a fourth aspect of the present invention, the second lens in thefirst lens group is formed of a plastic material whereas lens 2-1 in thesecond lens group is formed of a plastic material, and the zoom lenssystem further satisfies the following additional condition:

    (m.sub.2-2L -m.sub.2L).sup.2 <0.5                          (11)

In yet another aspect of the present invention, lens 2--2 in the secondlens group further satisfies the following additional condition:

    N.sub.2-2 <1.65                                            (12)

The symbols in conditions (1)-(12) have the following respectivedefinitions:

N_(1a-1) : the refractive index at the d-line of the first lens insubgroup 1a;

N_(1a-2) : the refractive index at the d-line of the second lens insubgroup 1a;

f₁ : the focal length of the first lens group;

f_(1a) : the focal length of subgroup 1a;

f_(1b) : the focal length of subgroup 1b;

Δ_(1a) : the amount of change in the coefficient of a third-orderspherical aberration caused by the aspheric surface in subgroup 1a(i.e., an aberrational coefficient as calculated on the assumption thatthe focal length of the overall system at the wide-angle end is 1.0);

m_(1a-2) : the lateral magnification of the second lens in subgroup 2a;

m_(1b) : the lateral magnification of subgroup 1b;

m_(2L) : the lateral magnification of the second lens group at thenarrow-angel end;

N₂₋₁ : the refractive index at the d-line of lens 2-1 in the second lensgroup;

f₂ : the focal length of the second lens group;

f₂₋₁ : the focal length of lens 2-1 in the second lens group;

m_(2-2L) : the lateral magnification of lens 2-2 in the second lensgroup at the narrow-angle end; and

N₂₋₂ : the refractive index at the d-line of lens 2-2 in the second lensgroup.

According to the invention, the first lens group may include a plasticlens having a negative refractive power, and the second lens groupincludes a plastic lens having a positive refractive power. Although itis well known to cancel the focus movement against the changes intemperature and humidity by making the negative lens and the positivelens of plastic, it is possible to cancel the ambient change within thelens system since the two lenses are likely to be arranged adjacent toeach other in the single lens group. However, according to the presentinvention, there is a unique arrangement such that these two lenses arearranged separately in the movable lens groups. Also, according to theinvention, the lenses of the different refractive powers (i.e., positiveand negative) are made of plastic, to thereby facilitate reduction ofrefractive powers of these lenses. If the power may be reduced, it ispossible to suppress the deterioration inherent in the plastic lensesdue to moulding errors and changes in environmental factors.Accordingly, it is possible to suppress the degradation in performancedue to the moulding error or ambient circumstance changes.

Also, according to another aspect of the present invention, the firstlens group may have a plastic lens interposed between glass lenselements, which leads to advantage that the system is free from anadverse affect of humidity. The plastic lens is likely to subjected tothe humidity, and hence the plastic lens should be sealed by glasslenses within a lens barrel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 3 and 5 are simplified cross-sectional views of the lenssystems of Examples 1, 2 and 3, respectively, at the wide-angle end; and

FIGS. 2A-2C, 4A-4C, and 6A-6C are graphs plotting the aberration curvesobtained with the lens systems of Examples 1, 2 and 3, respectively,with (a) showing the state at the wide-angle end, (b), the middle-angleend, and (c), the narrow-angle end.

DETAILED DESCRIPTION OF THE CONDITIONS

The zoom lens system of the present invention has a comparatively largeback focus in consideration of its use with a compact camera. Theoverall length of this lens system is significantly reduced and yetefficient compensation for aberrations can be achieved by usinginexpensive glass and plastic materials as well as aspheric lenssurfaces.

Conditions (1) and (2) relate to the optical materials of which thepositive first lens and the negative second lens in subgroup 1a aremade. If either one of these conditions is not satisfied, lensmanufacture at low cost cannot be realized.

Conditions (3) and (4) relate to the power distribution betweensubgroups 1a and 1bin the first lens group. These conditions requirethat subgroup 1a have a small power, with most of the power of the firstlens group being provided by subgroup 1b.

If condition (3) is not satisfied, the result is favorable for thepurpose of compactness but, on the other hand, a greater amount ofde-focusing or deterioration of lens performance will occur in responseto changes in temperature or humidity if a plastic material is used insubgroup 1a as will be described later in this specification.

Condition (4) is correlated to condition (3) and if its upper limit isexceeded, the positive power of subgroup 1b increases so much that notonly does it become difficult to compensate for spherical and chromaticaberrations at the narrow-angle end but also the likelihood of theoccurrence of higher-order aberrations will increase. If, on the otherhand, the lower limit of condition (4) is not reached, the result isfavorable for the purpose of effective compensation for aberrations butnot for the purpose of compactness.

Condition (5) relates to the aspheric surface in subgroup 1a. As alreadymentioned, subgroup 1a has a small power but by using an asphericsurface having a divergent amount of asphericity with respect to aparaxial radius of curvature, not only is it possible to achieveeffective compensation for spherical aberration within the first lensgroup but also the amount of change in spherical aberration that occursduring zooming can be reduced even if the power of subgroup 1a or itssize is reduced. The term "divergent" means that the radius of curvaturedecreases as the diameter of a concave surface increases whereas itincreases as the diameter of a convex surface increases. If the upperlimit of condition (5) is exceeded, the aspheric surface is notsufficiently effective in achieving satisfactory compensation foraberrations. If the lower limit of this condition is not reached,overcompensation will result to cause higher-order aberrations.

It is necessary here to add supplemental comments on the amount ofchange in the coefficient of a third-order aberration caused by anaspheric surface. An aspheric surface is generally expressed by thefollowing equation: ##EQU3##

In the case where the focal length, f, is 1.0, or if ##EQU4## A₁₀ =f⁹α₁₀ are substituted into said equation, the result is: ##EQU5##

The second and subsequent terms of this equation give the amount ofaspheric surface and the coefficient A₄ in the second term has thefollowing relationship with the coefficient of a third-order asphericityφ:

    φ=8(N'-N)A.sub.4

where N is the refractive index of a lens surface before it is madeaspheric and N' is the refractive index of the same surface after it isrendered aspheric.

The coefficient of a third-order asphericity introduces the followingamounts of change in the coefficients of various third-order aberrationsconsidered in the theory of aberrations:

    ΔI=h.sup.4 φ

    ΔII=h.sup.3 hφ

    ΔIII=h.sup.2 h.sup.2 φ

    ΔIV=h.sup.2 h.sup.2 φ

    ΔV=hh.sup.3 φ

where

I: the coefficient of spherical aberration;

II: the coefficient of coma;

III: the coefficient of astigmatism;

IV: the coefficient of truncated spherical field curvature;

V: the coefficient of distortion;

h: the height of intercept of each lens surface by paraxial, on-axisrays; and

h: the height of intercept of each lens surface by paraxial, off-axisrays passing through the center of the pupil.

The shape of an aspheric surface may be expressed in various other waysusing conicity coefficients or odd-numbered order terms and satisfactoryapproximation can be made using only even-numbered order terms if y issmaller than a paraxial radius of curvature. Hence, it should be notedthat one cannot depart from the scope of the present invention merely byusing equations for the shape of an aspheric surface that are differentfrom the one described above.

Another feature of the present invention is that if lenses of acomparatively small power in each of the first and second lens groups ismade of a plastic material, the amount of defocusing or deterioration inlens performance is small despite possible changes in temperature orhumidity. In addition, the overall weight of the lens system can bereduced. Further, it is easy to make an aspheric surface of plasticlenses and this contributes to an improvement in lens performance.

Supplemental comments are also necessary on the amount of defocusingwith plastic lenses that can occur in response to changes in temperatureor humidity. Plastics will experience temperature- or humidity-dependentchanges in linear expansion coefficient or refractive index that are atleast about 10 times as great as ordinary glass materials. If the amountof change in the focal length of a plastic lens is written as Δf, theamount of defocusing Δp can be expressed by:

    Δp=Δf(m'-m).sup.2

where m' is the lateral magnification of the lens groups exclusive ofand subsequent to the plastic lens, and m is the lateral magnificationof the combination of the plastic lens and subsequent lens groups.

Thus, in the case where the positive first lens in subgroup 1a is madeof a plastic material, m'=m_(1a-2) ·m_(1b) ·m_(2L) and m=0 and theamount of defocusing in response to changes in temperature or humiditywill become undesirably large if condition (6) is not met.

In the case where the negative second lens in subgroup 1a is made of aplastic material, m'=m_(1b) ·m_(2L) and m=m_(1a-2) ·m_(1b) ·m_(2L) andthe amount of defocusing in response to changes in temperature orhumidity will become undesirably large if condition (7) is not met.

In case that both of the positive and negative lenses in subgroup 1a areformed of a plastic material since the amounts of defocusing with thetwo lenses are opposite in sign and cancel each other. In this case,m'=m_(1b) ·m_(2L) and m=0 and the amount of defocusing in response tochanges in temperature or humidity will become undesirably large ifcondition (8) is not met. From the viewpoint of minimizing adverseeffects such as the deformation of plastic materials, it is desirablethat conditions (6) and (7) are also satisfied. It should, however, benoted that if the first lens in subgroup 1a is to be formed of a plasticmaterial, it is preferably protected with a coating or filter fromsurface flaws and other defects that impair the appearance of the lens.

The foregoing description concerns the first lens group but it should benoted that lenses in the second group may also be formed of glass orplastic materials that have low indices of refraction. Conditions (9)and (10) relate to positive meniscus lens 2-1 on the object side of thesecond lens group. If condition (9) is not met, an inexpensive zoom lenssystem which is one of the objects of the present invention cannot berealized. If condition (10) is not met, the power of lens 2-1 becomes sostrong that if it is made of a plastic material as described just below,the amount of defocusing or deterioration in lens performance inresponse to changes in temperature or humidity will become undesirablylarge. If condition (10) is not met, the positive power of lens 2-1becomes so small as to introduce difficulty in compensating forchromatic aberration that may occur within the second lens group.

If positive meniscus lens 2-1 is to be made of a plastic material, it ispreferred that the negative lens (second lens) in subgroup 1a is alsomade of a plastic material. If condition (11) is not met, the amount ofdefocusing in response to changes in temperature or humidity will becomeundesirably large.

For the purpose of realizing an inexpensive zoom lens system, the lastlens 2-2 on the image side of the second lens group may also be made ofa glass material of low refractive index that satisfied condition (12).

EXAMPLES

Examples 1-3 of the present invention are described below with referenceto data sheets, wherein f denotes the focal length, ω, half view angle,f_(B), back focus, r, the radius of curvature of an individual lenssurface, d, the thickness of an individual lens or the aerial distancebetween adjacent lens surfaces, N, the refractive index of an individuallens at the d-line, and ν, the Abbe number of an individual lens. In thefollowing data sheets, α₄, α₆ and α₈ signify the four-, six- andeight-order coefficients of asphericity.

EXAMPLE 1

    ______________________________________                                        F.sub.NO = 1:4.5 - 5.8 - 6.9                                                                    f = 39.02 - 50.00 - 60.00                                   ω = 28.5 - 23.3 - 19.8                                                                    f.sub.B = 13.87 - 25.08 - 35.30                             ______________________________________                                        Surface                                                                       No.    r         d             N      ν                                    ______________________________________                                        1      20.000    1.79          1.58904                                                                              53.2                                    2      26.641    1.27                                                         3      1771.623  1.20          1.58547                                                                              29.9                                                                          (plastics)                               4*    42.411    6.08                                                         5      70.000    3.08          1.56883                                                                              56.3                                    6      -10.315   1.38          1.80518                                                                              25.4                                    7      -13.414   8.83 - 5.27 - 3.17                                            8*    -25.453   1.90          1.58547                                                                              29.9                                                                          (plastics)                              9      -18.126   5.11                                                         10     -10.243   1.40          1.71700                                                                              47.9                                    11     -39.123                                                                ______________________________________                                        Fourth aspheric surface                                                                        Eighth aspheric surface                                      ______________________________________                                        α.sub.4 = 0.11170069 × 10.sup.-3                                                   α.sub.4 = 0.38552622 × 10.sup.-4                 α.sub.6 = 0.48768624 × 10.sup.-6                                                   α.sub.6 = 0.53665934 × 10.sup.-6                 α.sub.8 = 0.16049200 × 10.sup.-7                                                   α.sub.8 = -0.83225666 × 10.sup.-10               ______________________________________                                    

EXAMPLE 2

    ______________________________________                                        F.sub.NO = 1:4.5 - 5.8 - 6.9                                                                    f = 39.02 - 50.00 - 60.00                                   ω = 28.5 - 23.3 - 19.8                                                                    f.sub.B = 13.87 - 25.61 - 36.30                             ______________________________________                                        Surface                                                                       No.    r         d             N      ν                                    ______________________________________                                        1      20.000    1.82          1.51633                                                                              64.1                                    2      26.228    1.42                                                         3      -323.262  1.20          1.49186                                                                              57.4                                                                          (plastics)                               4*    50.830    6.39                                                         5      69.994    3.19          1.55963                                                                              61.2                                    6      -9.284    1.38          1.80518                                                                              25.4                                    7      -13.135   9.50 - 5.50 - 3.13                                            8*    -26.547   1.90          1.58547                                                                              29.9                                                                          (plastics)                              9      -17.852   4.84                                                         10     -9.951    1.40          1.58313                                                                              59.4                                    11     -65.802                                                                ______________________________________                                        Fourth aspheric surface                                                                          Eighth aspheric surface                                    ______________________________________                                        α.sub.4 = 0.11530464 × 10.sup.-3                                                     α.sub.4 = 0.44356893 × 10.sup.-4               α.sub.6 = 0.26590589 × 10.sup.-6                                                     α.sub.6 = 0.29611274 × 10.sup.-6               α.sub.8 = 0.15997148 × 10.sup.-7                                                     α.sub.8 = 0.27092752 × 10.sup.-8               ______________________________________                                    

EXAMPLE 3

    ______________________________________                                        F.sub.NO = 1:4.5 - 5.8 - 6.9                                                                    f = 39.00 - 50.00 - 60.00                                   ω = 28.6 - 23.3 - 19.9                                                                    f.sub.B = 13.34 - 24.80 - 35.21                             ______________________________________                                        Surface                                                                       No.    r         d              N     ν                                    ______________________________________                                        1      20.986    1.500          1.49186                                                                             57.4                                                                          (plastics)                               2*    29.028    2.063                                                        3      -29.571   1.217          1.58547                                                                             29.9                                                                          (plastics)                              4      -56.743   6.448                                                        5      57.586    3.490          1.70154                                                                             41.2                                    6      -9.315    1.300          1.80518                                                                             25.4                                    7      -19.599   9.370 - 5.468 - 3.163                                         8*    -21.660   1.900          1.58547                                                                             29.9                                                                          (plastics)                              9      -15.477   4.882                                                        10     -10.798   1.400          1.71300                                                                             53.8                                    11     -48.284                                                                ______________________________________                                        Fourth aspheric surface                                                                          Eighth aspheric surface                                    ______________________________________                                        α.sub.4 = 0.47598226 × 10.sup.-4                                                     α.sub.4 = 0.28932129 × 10.sup.-4               α.sub.6 = -0.79170457 × 10.sup.-7                                                    α.sub.6 = 0.81877936 ×  10.sup.-7              α.sub.8 = 0.80726584 × 10.sup.-8                                                     α.sub.8 = 0.31988019 × 10.sup.-8               ______________________________________                                    

Shown below are the values that are calculated for conditions (1)-(12)in each of Examples 1-3.

    ______________________________________                                        Condition Ex. 1        Ex. 2    Ex. 3                                         ______________________________________                                        (1)       1.589        1.516    1.492                                         (2)       1.585        1.492    1.585                                         (3)       0.117        0.102    0.055                                         (4)       1.127        1.111    1.066                                         (5)       -25.3        -22.4    -10.1                                         (6)       glass        glass    0.171                                         (7)       0.590        0.412    0.293                                         (8)       glass +      glass +  0.016                                                   plastics     plastics                                               (9)       1.585        1.585    1.585                                         (10)      0.259        0.320    0.323                                         (11)      0.187        0.263    0.253                                         (12)      1.717        1.583    1.713                                         ______________________________________                                    

According to the present invention, it is possible to relativelyincrease the back focus as a zoom lens for a compact camera and toreduce the overall length of the system. In addition, it is possible touse an inexpensive material of glass and/or plastic with insuring a highperformance.

What is claimed is:
 1. In a zoom lens system for use with a compactcamera that comprises, in order from the object side, a first lens grouphaving a positive focal length and a second lens group having a negativefocal length and which performs zooming by changing the distance betweenthe first and second lens groups, the improvement wherein said firstlens group comprises, in order from the object side, a subgroup 1a of asmall power that is composed of a positive first lens and a negativesecond lens and a subgroup 1b of a large positive power that has asurface of a negative power on the image side, said subgroup 1a havingat least one aspheric surface that has a divergent amount of asphericity with respect to a paraxial radius of curvature satisfying thefollowing condition (5), and said zoom lens system further satisfyingthe following conditions: ##EQU6## where: N_(1a-1) : the refractiveindex at the d-line of the first lens in subgroup 1a;N_(1a-2) : therefractive index at the d-line of the second lens in subgroup 1a; f₁ :the focal length of the first lens group; f_(1a) : the focal length ofsubgroup 1a; f_(1b) : the focal length of subgroup 1b; and Δ_(1a) : theamount of change in the coefficient of a third-order sphericalaberration caused by the aspheric surface in subgroup 1a (i.e., anaberrational coefficient as calculated on the assumption that the focallength of the overall system at the wide-angle end is 1.0);
 2. A zoomlens system according to claim 1 wherein the first lens in subgroup 1ais formed of a plastic material, with the following condition beingfurther satisfied:

    (m.sub.1a-2 ·m.sub.1b ·m.sub.2L).sup.2 <0.3 (6)

where: m_(1a-2) : the lateral magnification of the second lens insubgroup 1a; m_(1b) : the lateral magnification of subgroup 1b; andm_(2L) : the lateral magnification of the second lens group at thenarrow-angle end;
 3. A zoom lens system according to claim 1 wherein thesecond lens is subgroup 1a is formed of a plastic material, with thefollowing condition being further satisfied:

    (m.sub.1b ·m.sub.2L -m.sub.1a-2 ·m.sub.1b ·m.sub.2L).sup.2 <0.6                            (7)

where m_(1b) : the lateral magnification of subgroup 1b; m_(2L) : thelateral magnification of the second lens group at the narrow-angle end;and m_(1a-2) : the lateral magnification of the second lens in subgroup1a;
 4. A zoom lens system according to claim 1 wherein both the firstand second lenses in subgroup 1a are formed of a plastic material, withthe following condition being further satisfied:

    (m.sub.1b ·m.sub.2L).sup.2 <0.3                   (8)

where m_(1b) : the lateral magnification of subgroup 1b; and m_(2L) :the lateral magnification of the second lens group at the narrow-angleend;
 5. A zoom lens system according to claim 1 wherein the second lensgroup comprises, in order from the object side, a positive meniscus lens2-1 having a convex surface directed toward the image side and anegative meniscus lens 2-2 having a concave surface directed toward theobject side, which lens system further satisfies the followingconditions; ##EQU7## where: N₂₋₁ : the refractive index at the d-line oflens 2-1 in the second lens group;f₂ : the focal length of the secondlens group; and f₂₋₁ : the focal length of lens 2-1 in the second lensgroup;
 6. A zoom lens system according to claim 5 wherein lens 2-1 inthe second lens group is formed of a plastic material and satisfies thefollowing conditions:

    (m.sub.2-2L -m.sub.2L).sup.2 <0.5                          (11)

where m_(2-2L) : the lateral magnification of lens 2-2 in the secondlens group at the narrow-angle end; and m_(2L) : the lateralmagnification of the second lens group at the narrow-angle end.
 7. Azoom lens system according to claim 5 which further satisfies thefollowing condition:

    N.sub.2-2 <1.64                                            (12)

where N₂₋₂ : the refractive index at the d-line of lens 2-2 in thesecond lens group.
 8. A zoom lens system according to claim 5 whereinsaid negative second lens in the subgroup 1a of the first lens group isformed of a plastic material, and lens 2-1 in the second lens group isformed of a plastic material and satisfies the following condition:

    (m.sub.2-2L -m.sub.2L).sup.2 <0.5                          (11)

where m_(2-2L) : the lateral magnification of lens 2-2 in the secondlens group at the narrow-angle end; and m_(2L) : the lateralmagnification of the second lens group at the narrow-angle end.
 9. In atwo-group zoom lens system for use with a compact camera that consistsessentially of, in order from the object side, a first lens group havinga positive focal length and a second lens group having a negative focallength and which performs zooming by changing the distance between thefirst and second lens groups, the improvement wherein said first lensgroup comprises a plastic lens having a negative refractive power, andsaid second lens group comprises a plastic lens having a positiverefractive power.
 10. A zoom lens system according to claim 9, whereinsaid plastic lens having a positive refractive power is the only lens insaid second lens having a positive refractive power.
 11. A zoom lenssystem according to claim 9, wherein said plastic lens having a negativerefractive power is the only lens in said first lens group having anegative refractive power.
 12. A zoom lens system according to claim 9,said first lens group further comprising one or more plastic lenseshaving a negative refractive power, such that all said lenses in saidfirst group having a negative refractive power are made of plastic. 13.A zoom lens system according to claim 9, said second lens group furthercomprising one or more plastic lenses having a positive refractivepower, such that all said lenses in said second group having a positiverefractive power are made of plastic.
 14. A zoom lens system accordingto claim 9, wherein said first lens group further includes at least onelens of a positive refractive power that is made of glass, and saidsecond lens group further includes at least one lens of a negativerefractive power that is made of glass.
 15. In a zoom lens system foruse with a compact camera that comprises, in order from the object side,a first lens group having a positive focal length and a second lensgroup having a negative focal length and which performs zooming bychanging the distance between the first and second lens groups, theimprovement wherein said first lens group includes a first subgroup 1ahaving, in order from the object side, a positive first lens and anegative second lens, and as second subgroup 1b, said second lens groupincludes, in order from the object side, a positive lens 2-1 and anegative lens 2-2, said negative second lens of said first subgroup 1aof said first lens group and said positive lens 2-1 of said second lensgroup being made of plastic, said system meeting the followingconditions:

    (m.sub.1b ·m.sub.2L -m.sub.1a-2 ·m.sub.1b ·m.sub.2L).sup.2 0.6.                            (7)

    (m.sub.2-2L -m.sub.2L).sup.2 <0.5                          (11)

where m_(1a-2) : the lateral magnification of the second lens insubgroup 1a; m_(1b) : the lateral magnification of subgroup 1b; m_(2L) :the lateral magnification of the second lens group at the narrow-angleend; and m_(2-2L) : the lateral magnification of lens 2-2 in the secondlens group at the narrow-angle end.
 16. In a two group zoom lens systemfor use with a compact camera that consists essentially of, in orderfrom the object side, a first lens group having a positive focal lengthand a second lens group having a negative focal length, whereinthecamera performs zooming by changing the distance between said first andsecond lens groups, said first lens group comprises a plastic lensinterposed between two glass lenses, and said plastic lens and saidglass lenses are disposed within a lens barrel such that said glasslenses substantially protect said plastic lens from humidity.